First, a brief introduction about how inspiration occurs and how a ventilatory assist affects lung expansion will be provided.
Inspired lung volume or inflation of lungs is determined by the pressure distending the lungs, which is called the transpulmonary pressure PTR, and the mechanical properties of the lung, such as elastance and resistance of the lung. PTR is generated by the respiratory muscles, which through an outward action acts to expand the lungs. In respiratory failure, increased load or inspiratory muscle weakness results in an inability to adequately ventilate the lungs such that ventilation will become inefficient. In spontaneously breathing patients, addition of mechanical ventilation (artificial respiration) is used to aid the (presumably weak) respiratory muscles to overcome the increased inspiratory load. The level of ventilatory assist is currently determined rather arbitrarily with a major focus to restore adequate blood gases. In spontaneously breathing patients, the ventilatory assist level should be high enough to ensure that adequate ventilation can take place, however, one should avoid too high levels of ventilatory assist since this may result in disuse atrophy of the inspiratory muscles. There are currently no methods available to monitor and ensure that ventilatory assist levels are adequate.
Using a neurally controlled ventilator, that is a ventilator that responds to patients' neural effort in both time (triggering and termination of assist) and space (magnitude of assist) (as disclosed in U.S. Pat. No. 6,588,423 B1, granted to Sinderby et al. on Jul. 8, 2003, entitled “Method and Device responsive to Myoelectrical Activity for Triggering Ventilatory Support”), ventilatory assist is uniquely synchronized to patient effort and the mechanical ventilator could be considered as an additional artificial inspiratory breathing muscle under the influence of the brain's respiratory centers and neural respiratory feedback systems. Given the neural integration of such a system, it is not possible to set the assist or ventilation to too high values. Consequently, the system can unload muscles, improve ventilation to levels that are preferred by the patient's respiratory centers. However, a neurally controlled ventilator system resists “over assist” of the patient. Therefore, muscle unloading only takes place by overcoming inertia, elastic and resistive loads. Unlike the conventional systems (“not neurally controlled in time and space”), it is not possible to hyperventilate to very low breathing frequencies or apnea, such that the respiratory drive and respiratory muscle activity, due to chemo receptor influence, will always persist (by Sinderby et al., Chest 2007 In Press).
In unhealthy lungs, some air sacs may collapse, meaning that in those collapsed sacs, gas cannot enter or leave them, thus preventing gas exchange through the collapsed air sacs; in this case, a ventilator will supply a higher concentration of oxygen in order to provide proper blood oxygenation. Also, a ventilator can supply positive end-expirtory pressure (PEEP) to recruit or maintain airways open.
During the inflation process of the lungs, by increasing the transpulmonary pressure PTR, the collapsed air sacs will start to open up. When the collapsed air sacs start to open up, they are said to be recruited and the pressure at which the recruitment happens is called the critical opening pressure. However, continuing to increase the transpulmonary pressure PTR will lead to overinflation, which can be dangerous for the patient since it may cause lesions in the lung tissues, which will lead to air leakage out of the lung.
Furthermore, underinflation may also cause problems, such as atelectasis, when the recruited air sacs are de-recruited at a pressure threshold referred to as the critical closing pressure. Therefore, proper pressure provided by the mechanical ventilator should fall inside the thresholds of overinflation and underinflation pressures. In U.S. Pat. No. 5,937,854, granted to Alex Stenzler, on Aug. 17, 1999 and entitled “Ventilator Pressure Optimization Method and Apparatus”, a method and apparatus for controlling the ventilation pressure are disclosed. By increasing incrementally the pressure, the lung volume is measured and then compared to a previous volume measure. If the increase in the lung volume is higher than 20% when compared to the past value, then the critical opening pressure has been reached. Therefore, the ventilatory apparatus will stop increasing the pressure. To measure the critical closing pressure, the pressure in the lungs is decrementally decreased and, at each decremental decrease, the lung volume is measured and then compared to the previous value. If a change in the volume of more than 20% is observed, then it means that the critical closing pressure has been reached. And the mechanical ventilatory assist machine stops decreasing the pressure. This method presents the drawback of depending on very slow inflations to measure a static pressure.
In Patent Application EP 1 295 620 A1, published on Mar. 26, 2003, by J. Björn, and entitled “A Method for Examining Pulmonary Mechanics and a Breathing Apparatus System”, a method and apparatus for examining the pulmonary mechanics in a respiratory system is disclosed. More specifically, the apparatus determines a flow, volume and pressure of the gas streaming through the respiratory system. Furthermore, the apparatus compares the measured/determined flow, volume and pressure with reference values set by an operator and then produces an error signal for adjusting accordingly the apparatus. This method depends on oscillations in patients who are not breathing spontaneously.
In Patent Application EP 1 204 439 A1, published on May 15, 2002, by C. Sinderby, and entitled “Target Drive Ventilation Gain Controller and Method”, a device for adjusting the degree of inspiratory assist, in relation to the patient's respiratory drive, representing a real need of the patient, is disclosed. This device first detects a signal representative of a respiratory drive, then compares this signal to a target drive and finally adjusts the gain of a controller of a lung ventilator in order to control the lung ventilator in relation to the respiratory drive. However, such a method of controlling inspiratory proportional pressure assist ventilation requires no knowledge of the mechanics of the lung, such as its elastance and resistance.
Therefore, until now, no dynamic measurements of the mechanics of the lungs have been proposed, using a respiratory neural drive for controlling a ventilator assist.